Backrest Strength Test For Type III Chair

- Apr 14, 2018 -

Backrest Strength Test - Static - Type lll (See Figures 6a through 6d) 

 

6.1 Applicability This backrest strength test shall be performed on Type lll chairs with backrests greater than 200 mm (7.9 in.) in height. 

 

6.2 Purpose of Test  The purpose of this test is to evaluate the ability of the chair to withstand stresses such as those caused by the user exerting a rearward force on the backrest of the chair. 

 

6.3 Test Setup 6.3.1 The chair shall be placed on a test platform in an upright position and the base shall be restrained from movement, but shall not restrict movement of the backrest or arms of the chair.  Figure 6d shows one acceptable method of restraining the chair. 6.3.2 If adjustable features are available, all adjustments shall be set at normal use conditions, except for height-adjustable pivoting backrests which shall have the pivot point set at its maximum height or 406 mm (16.0 in.) whichever is less.  6.3.3 After making the above adjustments, determine points 406 mm (16 in.) and 452 mm (17.8 in.) above the seat.  (See Figure 6a and Section 3.5).  Mark these points on the vertical centerline of the backrest. a) If the top of the load-bearing structure/surface of the backrest is greater than or equal to 452 mm (17.8 in.) above the seat, position the center of the form-fitting device (See Definition 2.10) 406 mm (16 in.) above the seat.   (See Figure 6b). b) If the top of the load-bearing structure/surface of the backrest is less than 452 mm (17.8 in.) above the seat, position the top of the form-fitting device even with the top of the load-bearing structure/surface.  (See Figure 6c).   


6.3.4 Attach a loading device (front push or back pull) to the horizontal center of the backrest as determined above.  With the backrest at its back stop position, apply a force that is initially 90 degrees ± 10 degrees to the plane of the backrest (see Figure 6d).  The force is not intended to be maintained at 90 ± 10 degrees throughout the loading of the backrest.  If applying the load with a cable and pulley system, the cable must initially be a minimum of 762 mm (30 in.) in length from the attachment point to the pulley.  The angle of the backrest plane may be determined by the angle of the plane of the front of the CMD upright. Note: Where the design of the chair does not allow the transfer of force(s) from the formfitting device to the load-bearing structure/surface, then a bridging device 38 mm to 102 mm (1.5 in. to 4 in.) in height may be used to span the width of the load-bearing structure/surface. For backrests with complex or varying contours, the plane of the backrest may be defined by the front of the CMD upright.    

 

6.4 Test Procedures 6.4.1 Functional Load   a) A force of 667 N (150 lbf.) shall be applied to the backrest at the backstop position for one (1) minute.  If the backrest/tilt lock mechanism will not accept the load due to gradual slipping of the adjustment mechanism during the load application, set the backrest to its most rearward (stopped) position, then apply the specified load(s). b) Remove the load.  6.4.2 Proof Load a)  A force of 1001 N (225 lbf.) shall be applied to the backrest at the backstop position for one (1) minute.  If the backrest/tilt lock mechanism will not accept the load due to gradual slipping of the adjustment mechanism during the load application, set the backrest to its most rearward (stopped) position, then apply the specified load(s). b)  Remove the load. 

 

6.5 Acceptance Level 6.5.1 Functional Load A functional load applied once shall cause no loss of serviceability to the chair.  6.5.2 Proof Load A proof load applied once shall cause no sudden and major change in the structural integrity of the chair.  Loss of serviceability is acceptable. 


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